About the Detector 3

The liquid scintillator in the balloon and the buffer oil are contained in a 18-meter diameter spherical stainless steel tank (3000m^3). On the surface, this tank looks like a large green gas tank existing everywhere. However,the difference from these gas tanks is that it is made of stainless steel to keep it from rust. This appearance was described as “looks like a watermelon” or “seems like an undersea base” in newspapers. And now, an inner surface area of the containment vessel is about 1000m^2, the size of 4 tennis courts. On the inner surface, 1879 of 50-cm diameter photomultiplier tubes (PMTs) are mounted densely as photodetectors.

Additionally, this array includes 1325 high specification PMTs newly developed by Tohoku University and Hamamatsu Photonics KK, which are so high sensitive enough to detect a single-photon. When light falls on the glass of the PMT front window, a single electron is released from the photocathode deposited on the inner surface of the glass made of a photoemissive material. The electron is amplified one to 10 million times inside the PMT and collected as an electric signal by the signal cable. Then, the electric signal is sent to the electronics outside of the stainless steel tank with the signal cable. The pulse height of the electric signals is about 1mV (1mV = one thousandth of a volt).

Also, the photon arrival time can be precisely detected by the PMT, and its accuracy for a single photo-electron is about 1ns (1ns = one billionth of a second). In this time, even light, circumnavigat the Earth at the equator approximately 7.5 times in one second, would travel only 30cm. The position of neutrino reactions in KamLAND can be reconstructed from the relative times of PMT hits. One of the characteristic features of KamLAND is definitely a photodetector with so large and high performance PMTs as not be seen in any other place. Next